Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227 (“the '227 patent”), which is incorporated herein by reference in its entirety.
Spinal cord stimulation is a well-accepted clinical method for reducing pain in certain populations of patients. As shown in FIGS. 1A and 1B, a SCS system typically includes an Implantable Pulse Generator (IPG) 100, which includes a biocompatible case 30 formed of titanium, for example. The case 30 usually holds the circuitry and power source or battery necessary for the IPG to function. The IPG 100 is coupled to electrodes 106 via one or more electrode leads (two such leads 102a and 102b are shown), such that the electrodes 106 form an electrode array 110. The electrodes 106 are carried on a flexible body 108, which also houses the individual signal wires 112a-112p, coupled to each electrode. The signal wires 112 a-112p are connected to the IPG 100 by way of an interface 115, which may be any suitable device that allows the leads 102 (or a lead extension, not shown) to be removably connected to the IPG 100. Interface 115 may comprise, for example, an electro-mechanical connector arrangement including lead connectors 38a and 38b configured to mate with corresponding connectors on the leads. In the illustrated embodiment, there are eight electrodes on lead 102a, labeled E1-E8, and eight electrodes on lead 102b, labeled E9-E16, although the number of leads and electrodes is application specific and therefore can vary. The electrode array 110 is typically implanted along the dura of the spinal cord, and the IPG 100 generates electrical pulses that are delivered through the electrodes 106 to the nerve fibers within the spinal column. The IPG 100 itself is then typically implanted somewhat distantly in the buttocks of the patient.
As shown in FIG. 2, an IPG 100 typically includes an electronic substrate assembly 14 including a printed circuit board (PCB) 16, along with various electronic components 20, such as microprocessors, integrated circuits, and capacitors, mounted to the PCB 16. Ultimately, the electronic circuitry performs a therapeutic function, such as neurostimulation. A feedthrough assembly 24 routes the various electrode signals from the electronic substrate assembly 14 to the lead connectors 38a, 38b, which are in turn coupled to the leads 102 (see FIGS. 1A and 1B). The IPG 100 further comprises a header connector 36, which, among other things, houses the lead connectors 38a, 38b. The IPG 100 can further include a telemetry antenna or coil (not shown) for receipt and transmission of data to an external device such as a portable or hand-held or clinician programmer (not shown), which can be mounted within the header connector 36. As noted earlier, the IPG 100 usually also includes a power source, and in particular a rechargeable battery 26.
Also shown in FIG. 2 is an external charger 12 that is used to recharge the battery 26 in the IPG 100, which is explained in further detail below. The external charger 12 itself needs power to operate, and therefore may include its own battery 70, which may also be a battery that is rechargeable using a plug-in-the-wall holster (“cradle”) or power cord connection much like a cellular telephone. Alternatively, the external charger 12 may lack a battery and instead draw its power directly from being plugged into a wall outlet (not shown).
The external charger 12 can contain one or more printed circuit boards 72, 74, which contain the circuitry 76 needed to implement its functionality. In one embodiment, and as shown in FIG. 2, most of the circuitry 76 can be located on an orthogonal circuit board 74, which reduces interference and heating that might be produced by the charging coil 17 positioned on circuit board 72, as is further explained in U.S. patent application Ser. No. 11/460,955, filed Jul. 28, 2006. The external charger 12 also consists of a case or housing 15, typically formed of a hard plastic, which may be divided into top and bottom portions 15a and 15b. The case 15 can be hand-held, or body-worn, or portable. Junction 13 illustrates the location where the top and bottom portions 15a and 15b may be snapped together or connected by other means. Clamps 19 may be utilized to hold the circuit boards 72 and 74 in place mechanically. Clamps 19 are shown formed as a part of the bottom case portion 15b, although this is not strictly necessary, as other means can be used to stabilize the components within the case 15.
To wirelessly transmit energy 29 between the external charger 12 and the IPG 100, and as shown in FIG. 2, the charger 12 typically includes an alternating current (AC) coil 17 that supplies energy 29 to a similar charging coil 18 located in or on the IPG 100 via inductive coupling. In this regard, the coil 17 within the external charger 12 is wrapped in a plane which lies substantially parallel to the plane of the coil 18 within the IPG 100. Such a means of inductive energy transfer can occur transcutaneously, i.e., through the patient's tissue 25. The energy 29 received by the IPG's coil 18 can be rectified and used to recharge battery 26 in the IPG 100, which in turn powers the electronic circuitry that runs the IPG 100. Alternatively, the energy 29 received can be used to directly power the IPG's electronic circuitry, which may lack a battery altogether.
Inductive charging between the two coils 17 and 18 can produce significant heating in the external charger 12. Because the external charger 12 is in proximity with the patient's tissue 25, there is the risk that high temperatures in the external charger 12 could overheat (or burn) the skin of the patient. Accordingly, techniques have been proposed for controlling external chargers to ensure that safe temperatures are not exceeded. Usually, such techniques involve monitoring the temperature of the external charger by a thermocouple or thermistors. Should a threshold temperature be exceeded (Tmax), generation of the magnetic charging field at the external charger is temporarily suspended to allow the external charger time to cool. At some later point, perhaps once the temperature falls a few degrees below Tmax (i.e., to Tmin), charging can once again be enabled, with the process essentially duty cycling the charging coil 17 in external charger on and off, as shown in FIG. 3.
Despite such solutions, the inventor considers that further improvements can be made to the safety of external charger technology, and this disclosure provides one such solution, in which an external charger is controlled based on a pressure impingent on its case.